Somewhere in the course of human evolution, our European ancestors learned to stop loathing lactose and love the udder, and a new study sheds light on our sudden and mysterious love of milk.

NPR reports that geneticists in Sweden have traced the love story back to recent history. Even though one-third of Spaniards are more than a little tolerant of lactose (Manchego is exhibit A), geneticists found lactose intolerant genes in the bones of their ancestors just 5,000 years ago.

In other words, Spaniards seem to have experienced a sudden burst of lactose love, and this finding pokes a hole in a competing theory of our evolution. It was hypothesized that lactose tolerance spread among sun — starved Europeans in the north, hungry for vitamin D. They got their fix of D from cow’s milk, and the more milk they could digest, the more they thrived. So why would the gene proliferate in sun-drenched regions of the South?

Good question, and for scientists hard on the case, the plot has just thickened.